The invention relates to a process for preparing primary amines by hydrogenating nitrites in the presence of a supported catalyst comprising cobalt and optionally, in addition, nickel. The invention further relates to the use of this catalyst in a process for preparing primary amines by hydrogenating nitrites.
The hydrogenation of nitrites provides, depending on the reaction conditions and the catalyst used, primary, secondary or tertiary amines, although in general, a mixture of all three amine types is obtained. In the past, intensive research was carried out toward controlling selective hydrogenation to give one type of amine. The removal of secondary and tertiary amines from, for example, the desired primary amines requires additional apparatus and additional energy. It was found that improved selectivity to give primary amines may be achieved by adding ammonia to the reaction mixture. The hydrogenation of dinitriles suffers from the problem of condensation reactions which lead to cyclic products or oligomers.
Selectivity of nitrile hydrogenation is also influenced by the nature of the catalyst. The nature of the catalyst also influences the temperature necessary for the hydrogenation of the nitrites. For instance, in particular in the case of nitrites which are unstable under severe conditions such as high temperatures or in the case of hydrogenation of nitrites where, for example, a certain isomeric ratio is desired, hydrogenation at very low temperatures under protective conditions is desirable. According to the prior art, the most suitable catalysts for hydrogenating nitrites at very low temperatures and with high selectivity are Raney nickel and Raney cobalt.
For instance, U.S. Pat. No. 4,721,811 relates to a continuous process for preparing linear polyamines by hydrogenating polynitriles. The catalyst used is granular Raney cobalt which is obtainable by leaching of corresponding nuggets of the alloy.
U.S. Pat. No. 5,869,653 also relates to a process for catalytically hydrogenating nitrites. The catalyst used is a Raney cobalt catalyst and the hydrogenation takes place in the presence of catalytic quantities of lithium hydroxide and water.
However, the use of Raney catalysts and the preparation thereof brings disadvantages. For instance, Raney catalysts are pyrophoric, so they may only be transported, stored and introduced into the reactors under liquid or with the exclusion of air. The preparation of Raney catalysts is also very complicated, since a starting alloy first has to be prepared by melting, which then has to be leached using considerable quantities of concentrated bases. Afterwards, washing with water results in strongly basic waste water which, for ecological reasons, has to be worked up in a complicated manner or disposed of.
Accordingly, there is a great interest in replacing the Raney catalysts by other, less problematically handled catalysts.
A possibility is the use of supported catalysts. However, according to the prior art, these generally have low selectivities. However, supported catalysts are also known for which selectivity improvement by doping or by the use of cocatalysts is suggested.
For instance, EP-A 0 566 197 relates to a process for preparing primary amines by hydrogenating mono- and/or dinitriles using hydrogen in the presence of nickel and/or cobalt catalysts on a support material, preferably in combination with at least one solid cocatalyst which is insoluble in the reacton medium and the catalyst and/or cocatalyst are substantially nonacid. According to the examples, it was found that the addition of alkali metals or of alkaline earth metals results in a selectivity increase at an only slightly reduced activity. However, these reactions take place at 130 or 140xc2x0 C., i.e. under conditions which are unsuitable for thermally labile nitrites.
The use of a cocatalyst leads to increased complication, in particular when alkali metals or alkaline earth metals are used.
EP-A 0 424 738 relates to a process for preparing linear triamino compounds by reacting 1,3,6-tricyanohexane over a catalyst comprising cobalt oxide and also an oxide of alkali metals, alkaline earth metals, rare earths or scandium or yttrium. The catalyst may be unsupported or supported. The catalyst has insufficient activity which is noticeable by low space-time yields and the severe hydrogenation conditions (in particular, pressure of 300 bar).
DE-A 1 518 118 relates to a process for hydrogenating nitrites which employs cobalt catalysts comprising magnesium oxide. These may be unsupported or supported catalysts. According to Example 1 of DE-A 1 518 118, the active catalysts used are those in which 55% of the cobalt is present as cobalt metal. According to the examples, hydrogenation is carried out at temperatures of from 100 to 140xc2x0 C. Accordingly, this process is likewise unsuitable for hydrogenating thermally labile nitrites.
WO 97/10052 relates to a process for hydrogenating nitrites which employs hydrogenation catalysts comprising at least one divalent metal in partially reduced form (preferably Ni or Co) and at least one doping metal selected from the group consisting of Cr, Mo, Fe, Mn, Ti, V, Ga, In, Bi and rare earths in oxidic form. WO 97/10052 also relates to the catalysts themselves. The catalysts disclosed by WO 97/10052 are unsupported catalysts which consist predominantly of the active metal and are accordingly very expensive.
It is an object of the present invention to provide a process for preparing primary amines by hydrogenating nitrites which employs catalysts which are easy to handle and provide better value for money than unsupported catalysts, have high activities and selectivities for hydrogenating nitrites and are usable either as a powder in a suspension process or else as a shaped article in a fixed bed process. These catalysts shall also have high activity and selectivity toward primary amines even at low temperatures, so that the process is suitable for hydrogenating thermally labile nitrites.
We have found that this object is achieved by a process for preparing primary amines by hydrogenating nitrites in the presence of a catalyst comprising cobalt and optionally, in addition, nickel and also at least one further doping metal on a particulate support material.
The process according to the invention is characterized by the cobalt and, if present, the nickel having an average particle size of from 3 to 30 nm (nanometers) in the active catalyst.
Preference is given to the average particle size in the active catalyst being from 3 to 20 nm, more preferably from 3 to 15 nm, most preferably from 3 to 10 nm. The average particle size was determined by X-ray diffraction (Siemens D5000 diffractometer, TOPAS evaluation software). This catalyst used according to the invention has high activities owing to its large active surface area and small crystals and is notable for its long lifetime. To achieve the high activities, even small quantities of the catalytically active species are sufficient which allows inexpensive catalysts to be obtained.
The catalysts used according to the invention are prepared by a process comprising the following steps:
a) coprecipitation of cobalt and optionally, in addition, nickel and also at least one further doping metal from a solution comprising the corresponding salts of the metals mentioned onto a particulate support material,
b) subsequent drying and/or calcining,
c) optionally, molding, and
d) reduction.
a) Coprecipitation of Cobalt and, if Present, Nickel and also at Least One Further Doping Metal
The catalysts used according to the invention are prepared by coprecipitation (mixed precipitation, coprecipitation) of their components onto a particulate support material. To this end, an aqueous salt solution comprising the catalyst components is generally admixed with stirring and heating at temperatures of from 20 to 100xc2x0 C., preferably from 40 to 80xc2x0 C., with an aqueous mineral base, preferably an alkali metal base, for example, sodium carbonate, sodium hydroxide, potassium carbonate, potassium hydroxide or ammonium carbonate, until the precipitation is substantially complete. Instead of a mineral base, an organic carboxylic acid which forms insoluble metal salts may also be used. An example of such an organic carboxylic acid is oxalic acid.
The type of metal salts used is generally not critical. Coprecipitation requires good water solubility of the salts used so that a criterion relating to the type of metal salts used is their good water solubility for preparing the relatively highly concentrated salt solutions. It is essential that the choice of the salts of the individual components only include salts of such anions which do not lead to problems, for example, owing to undesired precipitations or complex formation. Useful salts generally include the corresponding nitrates, acetates, formates, chlorides, sulfates and oxalates. The concentrations of the salt solutions are generally from 5 to 100%, preferably from 10 to 90%, more preferably from 25 to 75% of the maximum solubility of the salt or salts in water.
The coprecipitation may be carried out by any desired process known to those skilled in the art. In general, it is particularly advantageous to maintain constant conditions during the coprecipitation to obtain highly homogeneous precipitate. In one embodiment, the slightly basic solution of the mineral base at a desired pH of generally from 4 to 8, preferably from 5 to 7, more preferably from 5 to 6, is used as the initial charge and then the solutions of the appropriate metal salts (of cobalt, optionally of nickel and also of further doping metals), the slurried support material and further mineral base are added, preferably continuously, while keeping the pH constant. However, preference is given to using the support material at the desired pH of generally from 4 to 8, preferably from 5 to 7, more preferably from 5 to 6, as the initial charge and then adding the individual metal salt solutions of cobalt, optionally of nickel and of doping metals. Finally, it is also possible to use the support material together with the metal salts mentioned as the initial charge and then adding the mineral base until complete precipitation has occurred.
In general, the precipitation procedure is carried out at temperatures of from 20 to 100xc2x0 C., preferably from 40 to 90xc2x0 C., more preferably from 50 to 80xc2x0 C. Depending on the metal salts used, the pH is from 4 to 7, preferably from 5 to 6.
In a preferred embodiment, the reaction mixture after precipitation is stirred for a certain time, in general from 10 to 90 min, preferably from 30 to 60 min, at the temperature mentioned, optionally while blowing in air. This further ages the mixture and thereby eases filtration. Finally, preference is given to raising the pH by adding further mineral base to a value of from 7 to 9, preferably from 7.5 to 8.5, in order to achieve virtually complete precipitation.
After the precipitation process, the precipitates obtained are filtered off and washed with water to remove all anions. The exact procedure is known to those skilled in the art.
b) Drying and/or Calcining
After the precipitates are filtered off and washed with water to remove all anions, they are dried and/or calcined. Preference is given to first drying and then calcining. The precipitates are generally dried at temperatures of from 70 to 150xc2x0 C., preferably from 90 to 130xc2x0 C. Drying is customarily carried out in an oven or in a spray dryer.
In order to decompose the carbonates, hydroxides or other compounds obtained depending on the anions used, the optionally dried solid is calcined. Calcining is effected at temperatures of generally from 250 to 600xc2x0 C., preferably from 300 to 450xc2x0 C. Examples of useful calcining devices include tray ovens and belt calciners. Both drying and calcining may be carried out using a temperature program which involves effecting the drying or calcining in a plurality of stages at differing temperatures. In general, drying takes from 2 to 24 hours, preferably from 6 to 15 hours. Calcining is generally effected over a period of from 1 to 24 hours, preferably from 2 to 10 hours, more preferably from 2 to 5 hours.
c) Optionally, Molding the Powder Obtained
In a preferred embodiment of the process according to the invention, the catalysts used according to the invention are used in a fixed bed process. Use in a fixed bed process requires shaping of the powder obtained after calcining. The shaping may be effected by processes known to those skilled in the art, for example by tableting or kneading and extrusion. To this end, shaping assistants such as Tylose(copyright), Walocel(copyright), potato starch, polyvinyl alcohol, lactic acid, polyethylene oxide and nitric acid are used.
d) Reduction
In the case of catalysts for use in fixed bed processes, shaping is followed by reduction of the catalyst precursors obtained. In a further embodiment of the process according to the invention, the catalysts are used as powders in a suspension process. In this case, no shaping takes place, and the reduction instead directly follows the calcining (step b).
Reduction is carried out using hydrogen at elevated temperatures of generally from 150 to 800xc2x0 C., more preferably from 200 to 600xc2x0 C. Preference is given to carrrying out the reduction of the cobalt catalysts used according to the invention in a plurality of stages. The first reduction stage is generally carried out at from 150 to 450xc2x0 C., preferably from 175 to 400xc2x0 C., more preferably from 200 to 350xc2x0 C., for from 1 to 24 hours, preferably from 1.5 to 12 hours, more preferably from 2 to 6 hours. The second reduction stage takes place after heating to from 300 to 650xc2x0 C., preferably from 350 to 550xc2x0 C., more preferably from 400 to 500xc2x0 C. This generally takes from 2 to 48 hours, preferably from 4 to 20 hours, more preferably from 6 to 15 hours. The second reduction stage may optionally be followed by a third reduction stage which is carried out in a range from 450 to 800xc2x0 C., preferably from 475 to 700xc2x0 C., more preferably from 500 to 600xc2x0 C. over a period of from 0.5 to 10 hours, preferably from 0.5 to 8 hours, more preferably from 1 to 6 hours.
The reduction is generally carried out at a pressure of from 0.1 to 300 bar, preferably from 0.1 to 100 bar, more preferably from 1 to 10 bar. In a highly preferred embodiment, the reduction is carried out at atmospheric pressure.
A significant factor in obtaining the catalysts used according to the invention is the ratio of hydrogen used for the reduction to the catalyst quantity. Preference is therefore given to carrying out the reduction at a ratio of hydrogen to the catalyst quantity of  greater than 1 Nm3/hxc3x97kgcat., preferably of  greater than 2 Nm3/hxc3x97kgcat. Particular preference is given to employing a ratio of hydrogen quantity to catalyst quantity of from 1 to 10 Nm3/hxc3x97kgcat., and very particular preference to a ratio of from 1 to 5 Nm3/hxc3x97kgcat.
The water partial pressure in the hydrogen which on the industrial scale is generally circulated is generally xe2x89xa6200 mbar, preferably from 0 to 200 mbar, more preferably from 0 to 100 mbar.
With the aid of this reduction step according to the invention, highly active catalysts are obtained which are notable for the particularly small cobalt and, if present, nickel crystal sizes and also a particularly high degree of reduction. The catalysts obtained in this manner are accordingly highly active and particularly suitable for the process according to the invention.
The catalysts obtained after reduction (d) are notable in that in general, at least 50%, preferably at least 65%, more preferably at least 70%, most preferably at least 75% of the cobalt and, if present, nickel in the catalyst are present in reduced form. This means that the catalysts used according to the invention comprise a particularly high fraction of active species and are therefore highly active even at low temperatures. The cobalt and, if present, the nickel in the catalysts used according to the invention are either cubic or hexagonal.
The doping metal present in the catalyst used according to the invention is generally selected from the group consisting of metals of groups IIIB, IVB, VB, VIB, VIIB, IIA, IIIA and VIA and the lanthanide group of the periodic table. Preference is given to using doping metals selected from the group consisting of metals of groups IIA, IIIB, IVB, IIIA, IVA and the lanthanide group. Particular preference is given to using doping metals selected from the group consisting of Ti, Zr, La, Y, Gd, Ce, Si, Al, Ga and Mg. With the aid of these doping metals, catalysts can be obtained which, as well as a high activity, have outstanding selectivities in the process according to the invention. It is possible for the catalysts used according to the invention to comprise at least one further doping metal selected from the group consisting of silver, gold and ruthenium, in addition to the doping metal selected from the groups mentioned.
The particulate support material may generally be any desired inert support material. For example, support materials may be selected from the group consisting of silicon dioxide, aluminum oxide, titanium dioxide, zirconium dioxide, silicon dioxide/aluminum oxide, aluminosilicates, calcium oxide, magnesium oxide, pumice and carbon and mixtures thereof. Preference is given to using support materials selected from the group consisting of silicon dioxide, aluminum oxide, titanium dioxide, zirconium dioxide, silicon dioxide/aluminum oxide, magnesium oxide and aluminosilicate. Very particular preference is given to using silicon dioxide and alumosilicates and also aluminum oxide and silicon dioxide/aluminum oxide and magnesium oxide as particulate support material.
In general, the doping metal is used in a quantity of from 1 to 90% by weight, preferably 5 to 70% by weight, more preferably from 10 to 50% by weight, based on the quantity of cobalt and, if present, nickel used. The mass ratio of cobalt and, if present, nickel to the sum of the support material and coprecipitated doping metal in oxide form is generally in the range from 20:80 to 80:20, preferably from 40:60 to 70:30. If nickel is present in the catalyst used according to the invention, the mass ratio of cobalt to nickel is in the range from 90:10 to 99.9:0.1, preferably from 80:20 to 99:1. The mass ratio of support material to coprecipitated doping metal in oxide form is generally in the range from 98:2 to 30:70, preferably from 90:10 to 50:50. The mass ratio of cobalt and, if present, nickel to silver, gold or ruthenium is generally in the range from 90:10 to 99.5:0.5.
The catalysts obtained in this way are outstandingly suitable for the hydrogenation according to the invention of nitrites to give primary amines under protective conditions.
The hydrogenation of nitrites may be carried out in different ways, and suitable process conditions are known to those skilled in the art. The hydrogenation may be carried out batchwise or continuously, although preference is given to a continuous method. Particular preference is given to carrying out the hydrogenation continuously in the liquid phase. The catalysts used in the process according to the invention are suitable in particular for continuous hydrogenation over a fixed bed or in suspension. When the hydrogenation is carried out over a fixed bed, either the trickle-bed method (downward flow through the reactor) or liquid phase method (upward flow through the reactor) may be employed. Useful reactors for hydrogenation over a solid bed include tube reactors. Particularly suitable embodiments of tube reactors are known to those skilled in the art.
When the hydrogenation is carried out in suspension, particularly useful reactors include stirred tanks, jet loop reactors or bubble columns.
However, it is also possible in principle to carry out the process according to the invention by batchwise operation. This involves adding the nitrile or a solution of the nitrile together with the catalyst into a high pressure autoclave, pressurizing with hydrogen and, if desired, ammonia and heating the reaction mixture. After the reaction has ended, the mixture is cooled, the catalyst removed and the reaction mixture fractionally distilled.
The hydrogen used in the hydrogenation is generally used in large stoichiometric excess. It may be recycled into the reaction as circuit gas. The hydrogen used is generally of technical purity. However, admixed inert gases, e.g. nitrogen, do not disturb the course of the reaction. The hydrogen pressure is generally from 0.5 to 30 MPa, preferably from 2.0 to 20 MPa, more preferably from 3.0 to 10 MPa.
The temperature in the process according to the invention is generally from 25 to 125xc2x0 C., preferably from 25 to 100xc2x0 C., more preferably from 50 to 100xc2x0 C. Comparatively low temperatures during the process according to the invention enable the hydrogenation of thermally labile nitrites and also the achievement of defined isomeric ratios, for example in the case of isophoronediamine. Even at such low temperatures, the catalyst used according to the invention is very good owing to the small average particle size of cobalt and, if present, the nickel and the resulting large surface area.
It is possible to add organic solvents in the process according to the invention by hydrogenating nitrites. This is sensible when the nitrile is, for example, solid under normal conditions. Preferred organic solvents include aliphatic alcohols, e.g. methanol or ethanol, amides, e.g. N-methylpyrrolidone and dimethylformamide, ethers, e.g. dioxane or tetrahydrofuran and also esters. Particular preference is given to using tetrahydrofuran as solvent.
The catalyst hourly space velocity is generally in the range from 0.5 to 20 mol of nitrile/lcatalystxc3x97h, preferably from 2 to 10 mol of nitrile/lcatalystxc3x97h. When di- or polynitriles are hydrogenated and only a partial conversion is desired, the conversion and product ratio may be adjusted by changing the residence time.
It is possible to improve the selectivity toward primary amines by adding ammonia. However, outstanding selectivities are achieved even without adding ammonia. When ammonia is used, the quantity of ammonia is generally in the range from 6 to 60 mol of ammonia per mole of nitrile, preferably from 12 to 36 mol of ammonia per mole of nitrile.
In principle, all nitrites can be converted by the process according to the invention to primary amines. Useful nitrites include both mononitriles and dinitriles and also polynitriles having more than two nitrile groups. Preference is given to converting mononitriles or dinitriles into the corresponding amines. The process according to the invention is particularly suitable for the hydrogenation of Strecker nitrites which are obtainable by addition of formaldehyde and hydrocyanic acid to nucleophilic centers such as amines; of Michael nitrites which are accessible by addition of acrylonitrile to nucleophilic centers such as amines; of iminonitriles and also of dinitriles, in particular xcex1, xcfx89-dinitriles.
Particular preference is therefore given to using nitrites of the general formulae (I), (II), (III) or (IV) in the processes according to the invention:
R13-nX-A-CNxe2x80x83xe2x80x83(I) 
where the symbols are defined as follows:
X is O, S, N, P or C, preferably O, N or C
A is xe2x80x94(CR2R3)mxe2x80x94, where R2 and R3 are each independently substituted or unsubstituted aryl, alkyl (branched or unbranched) or cycloalkyl radicals or hydrogen and m is from 0 to 8, preferably from 1 to 6, more preferably 1 or 2, or
is a C2- to C8-alkylene chain, where the alkylene chain is interrupted by from 1 to 4, preferably from 1 to 3, more preferably 1 or 2 nonneighboring heteroatoms, or
is phenylene, cyclohexylene, optionally substituted by radicals containing substituted or unsubstituted aryl, alkyl (branched or unbranched) or cycloalkyl radicals or by heteroatoms, preferably oxygen or nitrogen;
n is 2 (when X=O, S), 1 (when X=N, P) or 0 (when X=C);
R1 is a substituted or unsubstituted aryl, alkyl (branched or unbranched) or cycloalkyl radical or hydrogen, although when m=0 or 1, the R1 radicals may also be differing radicals selected from the R1 group, or two R1 radicals together with the heteroatom X (N, P, C) may form a cyclic radical which may be substituted in any desired manner and may be aromatic or cycloaliphatic;
NC-A-CNxe2x80x83xe2x80x83(II) 
where A is as defined above; 
where R4, R5, R6, R7, R8, R9, R10, R11 and R12 are each independently substituted or unsubstituted aryl, alkyl (branched or unbranched) or cycloalkyl radicals or hydrogen, and
o, p, q, r are 0, 1, 2, 3, 4 or 5.
Particular preference is given to using nitrites of the general formulae (Ia) or (IIa):
R3-nXxe2x80x94(CR2R3)mxe2x80x94CNxe2x80x83xe2x80x83(Ia) 
where
X is O or N,
m is 1 or 2, or
NCxe2x80x94(CR2R3)mxe2x80x94CNxe2x80x83xe2x80x83(IIa) 
where
R2 and R3 are each independently alkyl radicals (branched or unbranched) or hydrogen and m is from 2 to 4.
Preference is also given to iminonitriles of the general formulae III or IV.
Nitriles used with preference in the process according to the invention and the amines produced from them are shown in the following table:
Very particular preference is given to using nitrites selected from the group consisting of adiponitriles, imidodiacetonitrile, biscyanoethylpiperazine, dimethylaminopropionitrile and isophoronenitrilimine.
The present invention also provides the use of a catalyst according to the invention in a process for preparing primary amines by hydrogenating nitrites. Preferred embodiments of the catalyst and also of the hydrogenation process and the suitable nitrites have already been discussed.
The invention is illustrated by the following examples: